Still no answer from PayPal about the API error. They did respond to my inquiry about this error:

10002 Authentication/Authorization Failed - Internal Error

This error occurs because you need to set your permissions in your PayPal account. In order for a third party to make API calls on your behalf, you have to explicitly grant them permission to do so.
I could not find any instructions from the zen-cart instructions for this. So I think this is just a lazy answer?

I sent a follow-up about the API error - which they have so far ignored.